When transmitting images from a server or from an emitter to a receiver, or when storing images on a storage device, it is often useful to transmit and/or store a color transform associated with these images, in order to be able to further transform these images using this associated color transform.
Color transforms are commonly applied to images or parts of these images, whenever color coordinates of their colors need for instance to be adapted to another color encoding standard (colorimetric color management, Electro-Optical Transfer Functions—EOTF, Optic-Electrical Transfer Functions—OETF), and/or to another color gamut (color gamut mapping), and/or to another viewing conditions (color appearance) and/or to another artistic intent (color grading). When applied to colors of an image, a color transform can modify the luminance, the hue and/or the saturation of these colors, the contrast and/or the white balance, the black level and/or the white level, or other aspects of this image. A color transform can be a Color Look Up Table (CLUT), a splines-based model, a Gain-Offset-Gamma (GOG) model or any other parametric model.
Color gamut mapping is notably used for rendering images on image display devices, notably when the color gamut of a content to display (source color gamut) is different from the color gamut of the display device (target color gamut) used for the rendering. As the target color gamut is generally specific to each type of display device, it is common to transmit an adapted color transform to a receiving equipment connected to the display device (or integrated to it) together with the content to display, in order to allow this receiving equipment to apply this adapted color transform to the received content before sending it to the display device for rendering. Such receiving equipment can be for instance a gateway or a set-top-box. This receiving equipment can also be part of a TV set, of a tablet or of a cell phone. When the source color gamut changes from a sequence of this content to another sequence of the same content to display, a specific adapted color transform should generally be applied to images of each sequence of this content: in such a situation in which a video content to display is divided in a plurality of main sequences of consecutive images, a color transform associated with each main sequence is transmitted to the receiving equipment together with the images of this main sequence. Such main sequences can correspond for instance to different scenes of a content, for instance an outdoor scene following an indoor scene, or to different parts of a content, for instance an advertisement following a sequence of a movie.
A similar situation may occur when images are exchanged between different post-production facilities: each image or sequence of images is exchanged with its specific color transform allowing rendering the same colors such as to let all image producers perceive the same colors whatever are their viewing conditions and/or their color perceptions.
When a content is transmitted to a receiving equipment and/or stored on a storing device, color transforms associated with this content need also to be transmitted and/or stored. That is why besides color data describing each image of the content (RGB data together with pixel position data of this image), i.e. besides content data, color metadata defining these associated color transform need also to be transmitted and/or stored.
There are generally two kinds of such color metadata: descriptive metadata and mapping metadata. These metadata are advantageously standardized as follows.
Descriptive metadata are for instance adapted to describe the color gamut of an image, of a sequence of images or of a content. See for instance IEC 61966-12-1, Multimedia systems and equipment—Colour measurement and management—Part 12: Metadata for identification of colour gamut (Gamut ID). When associated with a video content, such color gamut metadata define the color gamut for which this content was created. The gamut ID metadata identify for instance a set of 3D indexed faces building a gamut boundary description which is organized into three levels. Other descriptive metadata may provide information on the color primaries and on the dynamic range of the display device that was used to generate or to master images of a content. See for instance SMPTE ST2086.
Mapping metadata comprises notably data describing color mapping, color transform or change of color data to be applied to colors of images. To define a color transform, such metadata comprise for instance a set of six 1D Look Up Tables and one 3×3 linear Matrix. See for instance MPEG SEI message Color Remapping Information (CRI) in ITU-T Recommendation H.265|International Standard ISO/IEC 23008-2, “High Efficiency Video Codec”, Edition 2.0, October 2014.
Content data related to colors to render and metadata related to color transform to apply to these colors before rendering can be transmitted to a receiving equipment over different channels and these channels may not be synchronized. For example, metadata related to an image may be received earlier or later than content data which these metadata are associated with.
The application of a color transform associated with a main sequence of images to images of this main sequence generally generates a change in the colors of these images. In the process of rendering images of different consecutive main sequences that are transformed using a color transform associated with the main sequence to which these images belongs, the change of color look generated by the change of color transform from one main sequence to another one will not be generally perceived by a viewer when these two sequences correspond to different contexts and when the change of color transform is perfectly synchronized with the change of sequence. Different contexts mean for instance different scenes of the content, one being for instance indoor and the other one being then outdoor, or different parts of the content, one part belonging to a movie and another part being an advertisement. Even if the change of color look between two different consecutive main sequences due to the change of color transform is perceived by a viewer, this effect is usually desired since the color data and the color transform metadata are generally created with a specific artistic intent. Generally, the change of color transform is temporally aligned to the change of main sequence.
A problem arises when the change of color transform is not synchronized with the change of sequence. It may happen for instance when metadata defining a second color transform associated with a second sequence is received too late to be able to apply this second color transform to the first images of this second sequence before rendering them.
Such a problem notably arises when:
metadata defining the second color transform are transmitted after the beginning of the rendering of the second sequence;
processing of these metadata in order to get this second transform takes a long time before being ready to be applied to first images of the second sequence;
processing first images of the second sequence using this second color transform takes too much time before sending the corresponding transformed images to the display device for rendering.
When the change of color transform is not synchronized with the change of sequence from a first one to a second one, the change of color transform will occur during the ongoing display of the second sequence and may then be perceived by a viewer as an artefact.
An aim of the invention is to limit such visual artefacts during the rendering of video content divided in different sequences of images having each their specific color transform.
In order to solve this problem, it is proposed to build a transition sequence, notably from images of the beginning of the second sequence, and to change progressively the color transform applied to images of this transition sequence, notably from the first color transform associated with the first sequence to the second color transform associated with the second sequence. U.S. Pat. Nos. 7,355,606 and 7,042,464 disclose using fade transition effects to transit from one scene (or sequence) to another, in order to provide a smooth transition between these two scenes, and to avoid abrupt scene changes. In an embodiment, these documents disclose a method to adjust a gamma look up table to generate such transition effects: a special gamma look up table is applied to each image during the transition, which is calculated according to a blending between a usual gamma look up table and a target color, wherein the blending factor is a function of the elapsed time from the beginning of the transition effect.